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Overnight dexamethasone suppression test

Patient information A-Z

Why am I having this test?

This test is to see whether your body can appropriately ‘switch off’ production of the hormone cortisol. Uncontrolled cortisol production causes higher than normal cortisol levels and the loss of the natural daily rhythm of production of the hormone. This is part of Cushing’s Syndrome. Cortisol is produced by the adrenal glands and higher than normal levels can lead to a variety of symptoms and clinical problems. This test is used to either help or exclude a Cushing’s disease diagnosis.

What do I need to do before the test?

Females only

  • If you are on the contraceptive pill that contains oestrogen (COCP) or oral hormone replacement therapy (HRT) containing oestrogen it may be necessary to stop this medication six weeks prior to the test as it may interfere with the interpretation of the test result. Your endocrine consultant will tell you if this is necessary in your case.
  • You will need to use an alternative form of contraception if you stop the COCP.
  • If you are pregnant or think that you are pregnant, please let us know and do not proceed with the test without discussion.

All patients

  • If you are taking steroid tablets or using steroid creams, steroid nasal sprays or steroid inhalers, please do not use them for 24 hours before the test as they may interfere with the test results. If you feel that this is not going to be possible, or you are unsure about stopping any medication, then please discuss this with us.
  • You may eat and drink normally before the test.
  • Please continue to take any medication you normally take unless this falls into the above mentioned categories.

What do I do for the test?

  • Please make sure you have booked a blood test for the next morning (day 2) before taking the tablets (day 1).
  • Take the prescribed 1 mg dose of dexamethasone. This is usually dispensed as two 0.5 mg tablets which are then taken together, at approximately 11pm (day 1).
  • Most patients tolerate the tablets without any side effects and they can be taken with water or juice. Some patients prefer to take them with milk as this may help prevent any indigestion / heartburn-like symptoms.
  • You must attend for a blood test at 9am the morning after you have taken the dexamethasone tablets (day 2). This should be booked in advance and needs to be at 9am for the test to be interpretable. This will be explained on the blood test form we provide.
  • You can attend the blood testing clinics at Addenbrooke’s Hospital (Cambridge University Hospitals) for the 9am blood tests or book a specific blood test appointment with your GP practice. Alternatively you can use a ‘walk in service’ at a local hospital or other facility near you. Please use the blood test form provided if you choose to go to your GP practice or a ‘walk in service’. Please let us know when and where you have had this blood test so we can obtain the results.

What happens during the test?

Unless any other tests have been requested at the same time, you will require a single blood tube sample to test for cortisol levels. Please note that if you have your test at Addenbrooke’s then the request will be on the computer system, though it may still be useful to have the request form with you if you have one.

A specialist endocrine nurse is responsible for organising these tests and is available for any questions you may have via the contact details below.

Are there any side effects to this test?

Usually there are none. Rarely, people may experience heartburn after taking the dexamethasone tablets. Dexamethasone can in theory lead to higher blood sugars in patients with diabetes, though in practice the single 1 mg dose is unlikely to have a significant effect.

How long will the test take?

The test starts when you take the dexamethasone and lasts until you have had the blood test the following morning.

What happens after the test?

  • If you have been asked to stop any medications prior to the test, the specialist nurse will advise when to start taking them again.
  • Based on the results of the test, your doctor or the specialist nurse will be able to decide whether any further tests are required.

When will I get the results?

The results will be reviewed by the endocrine consultant. You, your GP and any referring specialist doctor will be informed of the results when they have been reviewed.

Depending on the results of the blood test, the endocrine consultant may ask the nurse to contact you and arrange some more tests. Any further testing will be explained to you. You may be offered a clinic appointment to discuss the results of this test.

Contacts / Further information

Specialist nurses, Endocrine investigation unit – tel: 01223 217848

General information

Discounted parking is available for patients attending clinic appointments taking more than three hours. Please present your appointment letter to receive this discount.

Food and drink – there is a coffee shop in the treatment centre atrium and additional food and drink facilities in the main hospital concourse.

We are smoke-free

Smoking is not allowed anywhere on the hospital campus. For advice and support in quitting, contact your GP or the free NHS stop smoking helpline on 0800 169 0 169.

Other formats

Help accessing this information in other formats is available. To find out more about the services we provide, please visit our patient information help page (see link below) or telephone 01223 256998.

Contact us

Cambridge University Hospitals
NHS Foundation Trust
Hills Road, Cambridge

Telephone +44 (0)1223 245151